Power over Ethernet (PoE)

If your network will be of any size at all, power over Ethernet (PoE) is extremely important. When designing a wireless network, access points should be located where it makes sense to put them for radio propagation purposes, not where it makes sense to put them for electrical power purposes.

Access points sold in the first wave of products had to be powered like most other computing devicesthat is, plugged into a nearby AC power outlet. To make matters worse, many of them used large "wall wart" transformers that blocked adjacent outlets, too. Locating access points was not a matter of finding the best locations for providing service, but near nearby power outlets. Modifying access point locations can be challenging if one of the constraints is staying near electrical power.

Due to the dangers of electrical work, there are legal requirements to use licensed electricians in many locations. Some organizations may have also negotiated with unions to provide skilled electrical labor. In either case, modifying access point locations may be quite expensive. In many locations, data cable such as Ethernet can be moved or installed by network technicians, so APs can be installed or relocated by nonunion labor.[]

[] Some jurisdictions may define Ethernet as "low-voltage wiring," which requires that it be installed by an electrician. The city of Redmond, Washington, taxes low-voltage wiring as part of the building permit process.

Types of PoE

There are a number of types of power equipment on the market; many comply with the IEEE 802.3af standard. 802.3af defines a transmission standard that can supply 48 volts DC up to a maximum of approximately 15 watts at the full length-reach of an Ethernet cable. The standard itself can be downloaded from the IEEE Get802 site, and more information is available from http://www.poweroverethernet.com/.

The most basic distinction is which wires in the Ethernet cable supply the power. Data is always carried on the data conductors (wire pairs 1 and 2 and 3 and 6). Alternative A in the standard carries power on the data conductors, while Alternative B carries power on the unused pairs (wire pairs 4 and 5 and 7 and 8). When power is carried on the data cables, the voltage polarity may go either way, as shown in Figure 20-1. The corresponding pinout is in Table 20-1. Power carried on the unused data cables must have a specified polarity. Nearly all vendors use positive polarity on the first pair; the notable exception is early Cisco-proprietary power devices, which reversed polarity from most other devices.

Figure 20-1. Power wire map

Table 20-1. Wire usage in 802.3af

Wire number

Pair mapping

Alternative A (MDI-X)

Alternative A (MDI)

Alternative B

1

1 - data

-

+

2

1 - data

-

+

3

2 - data

+

-

4

3 (unused)

+

5

3 (unused)

+

6

2 - data

+

-

7

4 (unused)

-

8

4 (unused)

-

A further distinction between types of power devices is where the power is supplied. Power equipment may be present in the switch, which is referred to as endpoint power. Alternatively, it may be added into the wiring by a power injector, in which case it is called mid-span equipment. Most 802.11 installations to date have used power injectors, and therefore mid-span equipment. As switch vendors have come out with power blades and they have been much more widely purchased, endpoint injection is become much more common. Figure 20-2 illustrates the difference between the two options. The injector shown in the mid-span case may be a stand-alone device, or it may be a "power patch panel" with multiple ports. Gigabit data links can only be endpoint powered, although this is not yet an important distinction for 802.11.

Figure 20-2. Endpoint and mid-span power injectors

802.3af incorporates detection of powered devices to avoid frying equipment plugged into a power port. Early PoE equipment would supply the full voltage all the time, and had a propensity to destroy devices that were not designed to cope with power. For safety reasons, 802.3af includes a handshake that slowly ramps up power. If there is no response to the initial probe from the power equipment, it stops providing it to protect the device at the end.

Power injectors almost always have two copper Ethernet ports and a power cord. One Ethernet port is for the network connection, and the second provides power on the network connection on the first port. A notable exception is the Cisco AIR-PWRINJ-FIB, which provides power on an Ethernet port while using a 100BASE-FX fiber for network connectivity.